Fish are not immune to anything. Specially not ich or other parasites. Just like how we as humans are not 100% immune to anything. If everything is good in the tank, your fish develop a good immune system. But they can most certainly still get parasites and diseases.I have had minimum loss when introduce fish one at a time after a short observation period without quarantine. Not saying no loss, but very few. All my fish are immune to ich or other parasites. I have never had an established fish get ich/velvet. I had problem when I was in over my head bought 10 fish and put them all together I lost most of them. Aggression and stress is the biggest trigger of disease outbreaks.